Process for spraying liquiform materials



April 30, 1963 Tlal.,

/4 TTRA/EYS United States Patent O Mice 3,037,534 PRCESS FOR SPRAYINGLIQUIFORM MATERIALS Leander H. Walker, 23 The Crescent, Berkeley, Calif.Filed Aug. 17, 1959, Ser. No, 834,126 5 Claims. (Cl. 159-48) The presentinvention relates generally to the spraying of liquiform materials suchas concentrated food liquids, purees, and the like, and moreparticularly to a method of spraying such materials within a relativelyhigh vacuum system.

The spraying of liquiform materials at Vatmospheric pressure is commonlyaccomplished by means of air spray guns or the like wherein the liquidis forced under pressure through a central opening, jet, or nozzle andone or more air streams are directed upon the liquid issuing therefromto break up the liquid stream into minute particles. The air streams,moreover, direct the liquid particles in a desired pattern of spray upona surface to be coated and serve the important function, particularlyWhere a at spray pattern is produced, of insuring an equal depth ofcoverage over the entire pattern to enable the application of a uniformthickness of spray over a given width of continuously moving surface.Although compressed air satisfactorily accomplishes the foregoingfunctions for substantially all common applications where materials aresprayed in the open air, viz., at atmospheric pressure, I have foundsuch common and accepted practice involving the use of air in spray gunsvastly unsuited to the spraying of liquiform materials in a vacuumsystem maintained at an absolute pressure of less than about l5 mm. ofmercury. More explicitly, when liquiform materials, particularlyconcentrated food purees and juices, are introduced into an air spraygun for the purpose of spraying a flat pattern on a moving belt in sucha relatively high vacuum system, the amount of heat absorbed by theexpanding air in issuing from the gun is sufficient to cause growth ofwhiskers of frozen liquiform material at the nozzle tip and eventualfreezing of the entire tip area. Even when the air is preheated auniform fiat pattern can not be maintained for more than a few minutesat absolute pressures of less than about 12 mm. of mercury absolutebecause the air tends to dry out the feed material at the tip of thenozzle which therefore becomes coated with essentially dry material,grows whiskers and becomes plugged up in short order. Therefore, theconventional practice of utilizing air in a spray gun is unsatisfactorywhen it is desired to continuously produce a uniform coating of aliquiform material on a moving surface within a vacuum system maintainedat absolute pressures less than about l2 mm. Hg. Both the cooling anddrying effects utilizing air produce undesirable effects.

The present invention overcomes the above-noted difficulties encounteredin spraying liquiform materials within relatively high vacuum byproviding a method for spray ing liquiform materials within a vacuumsystem at ab solute pressures less than about three times the absolutepressure at which the solvent or suspending liquid of the liquiformmaterial freezes. More particularly, provision is made for the operationof a conventional spray gun in accordance with an improved methodwhereby the disadvantages and limiting factors encountered in theconventional operation of a spray gun in a relatively high vacuum areovercome and the range of reduced pressures in which successfuloperation is readily achieved is materially extended. Solvents orsuspending agents in addition to water are included within the genericscope of this invention and the liquiform materials are generallyliquiform suspensions including nely divided solids dispersed in a lluidmedium.

It is therefore an object of the present invention to 3,087,534 PatentedApr. 30, 1963 provide a method of continuously applying a uniformcoating of a water suspended liquiform material on a moving belt withina relatively high vacuum system even at absolute pressures as low as lmm. of mercury.

Another object of this invention is the provision of a method ofbreaking up a stream of liquiform material into minute particles andcontinuously directing the particles to form a desired pattern on asurface disposed in a relatively high vacuum at pressures below about 12mm. of mercury.

It is still another object of my invention to provide a method ofspraying liquiform material from a conventional spray gun in relativelyhigh vacuum without drying and freezing of the liquiform material at thetip of the nozzle of the gun.

Yet another object is the provision of a method of spraying a relativelyWet coating of liquiform material upon a surface disposed in relativelyhigh vacuum with a minimum of dusting of the sprayed material upon thesurface.

It is a further object of the present invention to provide a method ofspraying liquiform material in a vacuum system maintained at extremelylow absolute pressure which facilitates a material reduction in the sizeof the vacuum equipment.

The invention possesses other objects and features of advantage, some ofwhich, with the foregoing, will be set forth in the followingdescription of the preferred form of the invention which is illustratedin the drawing accompanying and forming part of the specification. It isto be understood, however, that variations in the showing made by thesaid drawing and description may be adopted within the scope of theinvention as set forth in the claims.

FIGURE 1 is a schematic flow diagram of preferred means for conductingthe spraying method in accordance with the present invention, and

FIGURE 2 is a sectional view of one type of tip which may be used withthe gun.

Considering now the invention in some detail and referring to thedrawing, there is provided a vacuum enclosure 11 housing a relativelylow pressure region 12 established by conventional vacuum equipment (notshown) associated with the enclosure. Within the enclosure there isprovided an endless belt '13 which continuously moves forward in thedirection depicted by the arrow with a constant rate of travel. Disposedadjacent the moving belt in spaced relation to the surface thereof Visdisposed a conventional spray gun 14 into which liquiform material, eg.,concentrated food purees or juices, is introduced by means of a pump 16,or equivalent means, for spraying upon the surface of moving belt 13 toproduce a uniform coating of the liquiform material thereon. Theconventional spray gun 14 typically includes a central opening, jet, ornozzle 17 through which the liquiform material is forced under pressureby the pump 16. lmmediately after leaving the nozzle, the stream ofliquiform material is struck by one or more streams of impelling vapor,preferably a fluid material in a superheated vaporous state, introducedto the gun through the gas inlet 18 thereof under pressure. The inlet 18communicates with an annular passage 19 disposed concentrically aboutthe nozzle 17. The stream or streams of vapor issuing from the passage19 are controlled and directed radially inward towards the opening ofnozzle 17 by means of a cap 21 located at the operating tip of the gun.The cap surrounds all but the tip of the nozzle and therefore the highvelocity vapor stream breaks up the liquiform material stream issuingtherefrom into minute particles. Such particles are directed from thetip of the cap 21 by the vapor issuing therefrom in the desired patternof spray, i.e., flat, round, oval, etc. for impingement upon a surfaceto be coated.

Although the spray gun tip detailed above is of the conventionalinternal mix type, tips of the external mix variety may also beemployed. Such spray gun tips are similar in overall operation except inthe external mix type the break up function takes place after the streamof liquiform material leaves the cap. In addition to the foregoing typesof spray guns other devices capable of performing a similar sprayforming function may also be employed.

As indicated previously herein, air has been heretofore utilized as theimpelling gas introduced to the inlet 18 of spray gun 14 or to otherwiseaccomplish the spray forming process when the liquiform material issprayed at atmospheric pressure. However, when the pressure in region 12is reduced to less than about l2 mm. of mercury, under no conditions cana uniform flat spray upon belt 13 be maintained for extended periods oftime. This is due to the cooling effect produced by the rapid expansionof the air in leaving the cap ports and entering the low pressure regionto thereby absorb a substantial quantity of heat from the liquiformmaterial issuing from the nozzle, and thus rapidly `freeze the nozzletip. To this is added the drying effect of the rapidly expending airupon the liquiform material issuing from the nozzle which causeseventual plugging of the tip.

In order to overcome the foregoing difficulties, I provide a method ofspraying liquiform material wherein a stream of the liquiform materialdirected into low pressure region 12 is broken up into a spray forimpingement upon the surface of belt 13 by the direction of at least onepressurized stream of vapor, preferably the suspending medium of theliquiform material in vapor form, upon the entering stream of liquiformmaterial. Where the spray nozzle 14 is employed to produce the spray,the vapor is directed under pressure to the inlet 18 of the gun. Thetemperature of the vapor, moreover, is best regulated at a temperaturewell above the freezing temperature of the liquid at the pressuremaintained in region 12. The differential between the pressure of theentering vapor and pressure in region 12 is preferably adjustedsufficiently great to achieve the desired pattern Vand breakup of theliquiform material stream and yet sufficiently small that the amount ofheat absorbed from the liquiform material by the vapor in expandingthrough the pressure differential is less than the amount of heat lossrequired to lower the temperature of the liquiform material to itsfreezing point. The liquiform material at or near the nozzle tip of gun14 is thereby prevented from freezing. In addition the moisture contentof the vapor is best regulated sufficiently great to keep the nozzle tipwet but not so great as to form droplets thereon and interfere with gunoperation.

The vapor for the accomplishment of the spraying method in accordancewith the present invention may be prepared by any one of various methodsand means therefor well known in the art. One highly flexiblearrangement by which water vapor, for example, can be prepared havingtemperature, pressure, moisture content, and the like variable over awide range to -facilitate spray operation of the gun 14 over a widerange of low pressures in region 12 in accordance with theconsiderations described hereinbefore is schematically depicted in thedrawing. In the illustrative example, water vapor is superheated by highpressure steam in a heat exchanger 22 in a readily adjustable manner andthe super heated vapor fed to inlet 18 of spray gun 14. Morespecifically, the high pressure steam is fed by conduit 29 to shell 31the enclosed space of which constitutes the hot side of exchanger 22 andthe condensate is removed from the high pressure steam at the hot sideof exchanger 22 as by passing the steam and condensate therefrom throughconduit 32 to a steam trap 23. The pressure of the steam is next reducedpreferably to the range of l to p.s.i.g. by feeding the steamy throughconduit 33 to a pressure reducing valve 24 or equivalent means.Condensate in the resulting low pressure steam is then removed bypassing same through conduit 34 to a trap 26 and the low pressure steamis then led to the cold side of heat exchanger 22 by means of conduitline 36 for superheating to the desired degree. The quantity of steam orwater vapor fed to the exchanger is controlled as by means of a valve 27inserted in the line between trap 26 and the cold side of the heatexchanger. The cold side of heat exchanger 22 constitutes theheader-tube bundle 37 within the shell 31 thereof wherein the lowpressure vapor absorbs heat from the steam in shell 31 and is therebyprovided with the desired degree of superheating. Superheated steam fromthe heat exchanger 22 is applied to the inlet 18 of gun 14 throughconduit line 39 in which is provided a control valve 28. With theforegoing arrangement for preparing superheated water vapor, the amount,pressure, temperature and degree of undersaturation of the water vaporis readily adjustable through a combination of settings of reducingvalve 24 and control valves 27 and 2S.

As an operating example of the method of the present invention, thevacuum system was set to establish a pressure of 5 mm. of mercuryabsolute in low pressure region 12. Tomato paste containing 30% solidsby weight was pumped to spray gun 14 by means of pump 16. Water vapor atatmospheric pressure (212 F. steam) from the cold side of heat exchanger22 was expanded through valve 28 to supply heated water vapor at about140 F. to the spray gun. The differential between the pressure of watervapor in the vapor feed line (approximately mm. of mercury) and that inthe low pressure region was suicient to provide a ow of vapor throughthe gas ports of spray gun 14 sufficient to break up the stream oftomato paste into minute particles and to direct these particles into aliat and uniform spray pattern on the moving belt 13. The vapor suppliedto the gun was observed to contain particles of liquid water due to theadiabatic expansion of the vapor taking place in valve 28. The nozzletip remained wet, no whiskers were formed and a wet, non-dusting spraywas obtained in continuous operation over a period of many hours.

In another operating example of the present invention, the vacuum systemwas `set to establish an absolute pressure of 2.5 mm. in region 12.Tomato paste containing 32% solids by weight was pumped to spray gun 14by means of pump 16. Water vapor at 14 p.s.i.g. and containing 12 F.superheat was supplied from the cold side of heat exchanger 22 andexpanded through valve 28 to supply heated Water vapor at about 160 F.to the spray gun. The water vapor supplied to the gun contained fewerdroplets of condensed water than in the preceding example, butsuilicient heat and moisture was supplied to the nozzle tip so that thetip remained wet, no whiskers were formed and a wet, non-dusting spraywas obtained in continuous operation over a period of many hours.

:In still another operating example of the present invention, the Vacuumsystem was set to establish an absolute pressure of 3.2 mm. in region12. Tomato paste containing 32% solids by weight was pumped to spray gun14 by means of pump 16. Water vapor at 7 p.s.i.g. and containing 20 F.superheat was supplied from the cold side of heat exchanger 22 andexpanded through valve 28 to supply heated water vapor at about 150 F.to the spray gun. The water vapor supplied to the gun contained fewerdroplets of condensed water than in the preceding example, butsufficient heat and moisture was supplied to the nozzle tip so that thetip remained wet, no whiskers were formed and a wet, non-dusting spraywas obtained in continuous operation over a period of many hours.

Similarly, at any given absolute pressure in region 12, it was foundthat several combinations of the settings of valves 24, 27, and 28 wouldproduce the desired type of pattern and continuous operation. The valvesare set to provide the necessary difierential pressure between the vaporentering the spray gun and the vacuum region to achieve the desiredpattern and stream breakup, suificient heat to prevent yfreezing ofmaterial at Vor near the nozzle tip in the 4form of heat carried in thevapor, and sutlcient moisture in the vapor to keep the nozzle tip wetrbut insuicient moisture to `form droplets and interfere with gunoperation,

It will be appreciated that the spraying method of the present inventionis -further advantageous in that with such a method the admission of airto the vacuum -system providing low pressure region `12 is eliminated.Vacuum systems primarily designed ifor the removal of solvent orsuspending liquid are conventionally provided with internal condensingsystems capable of condensing considerable volumes of solvent orsupending liquid but are provided with only enough air removal capacityto handle small air leaks. "Hence considerably larger equipment would berequired if air were used as the spraying gas.

What is claimed is:

1 In a process for continuously spraying a liquiform aqueous suspensionin a closed system evacuated to a lou subatmosp-heric pressure, thesteps comprising directing at least one jet stream of said suspensioninto an intermixing zone in communication with said closed system,directing a Stream of superheated steam into said intermiXing zone toimpin-ge upon, intermix with and heat said suspension stream, said steambeing superheated to a temperature sufficient to eliminate freezing ofthe suspension at least in the area of introduction into said closedsystem and being introduced at such a rate as to maintain asubatmospheric zonal vapor pressure in said intermiX-ing zone above thepressure in said closed system, and discharging the intermixed streamsfrom said intermixing zone through a restrictive orifice zone into saidclosed system to maintain said vapor pressure, said zonal vapor pressurethereby propelling and dispersing said suspension into said closedsystem to -form a spray stream of suspension droplets in water vaporwhererby drying of said droplets is minimized, wherein said reducedpressure of said closed system is below about l2 mm. Hg, the zonal vaporpressure in said intermixing zone is of the order of about 150 mm. Hgand said steam is superheated to a temperature of about 12 to 20 F.above the boiling point in said system.

2. In a process for continuously spraying a uniformly distributed layerof a liquiform suspension upon a surface in a `closed system evacuatedto a low sub-atmospheric pressure of about 12 mm. Hg absolute, the stepscornprising directing at least one jet stream of said suspension into anintermixing zone in communication with said closed system, the pressurein the intermixing zone being on the order of about 150 Hg, directing astream of superheated vapor comprising the superheated vaporized uidphase of said suspension into said intermixing zone to impinge upon,intermix with and heat said suspension jet stream7 said vapor comprisingthe vaporized fluid phase of said suspension being superheated to atemperature of about 12 to 20 F. above the boiling point in the systemsufficient to minimize freezing of the suspension in the area ofintroduction into said system and being introduced at such a rate as tomaintain a subatmospheric vapor pressure in said intermixing zone abovethe lpressure in said closed system, and discharging the intermixedstreams through a restrictive orice zone into said closed system tomaintain dierential vapor pressure therebetween thereby propelling anddispersing said suspension into said closed system to -form a spraystream of liquid droplets in vapor of said Huid phase whereby drying ofsaid droplets in the vicinity of said orifice zone is minimized, anddirecting said spray stream to deposit upon said surface in the closedsystem to deposit thereon as a distributed fluidic layer.

3. In a process Ifor continuously spraying a uniformly distributed layerof a liquiform suspension -food-stui upon a surface in a closed systemevacuated to a low subatmospheric pressure of below about 12 mm. Hgabsolute, the steps comprising directing at least one jet stream of saidsuspension into an intermixing zone in cornmunication with said closedsystem, directing a stream of vapor comprising the superheated vaporizedfluid phase of said suspension into said intermixing zone to impingeupon, intermix with and heat said suspension jet stream, said vaporbeing superheated to a temperature in the range of about 12 to 20 F.above the boiling point in said closed system sufcient to minimizefreezing of the suspension in the area of introduction into said systemand being introduced at such a rate as to maintain a subatmosphericvapor pressure in said intermixing zone above the pressure in saidclosed system, and discharging the intermixed streams through arestrictive orice zone into said closed system to maintain a dilerentialvapor pressure therebetween, thereby propelling and dispersing saidsuspension into said closed system to form a spray stream of liquidIdroplets in vapor of said iluid phase whereby drying of said dropletsis minimized in said area, and directing of said spray stream to depositupon said surface in the closed system to deposit thereon as adistributed fluidic layer.

4. The process as defined in claim 3 wherein said pressure in saidintermixing zone is of the order of about mm. Hg.

5. The process as dened in claim 3 wherein said spray stream is directedprogressively along said surface whereby a continuous tluidic film isuniformly applied upon said surface.

References Cited in the tile of this patent UNITED STATES PATENTS1,350,247 Stutzke Aug. 17, 1920 1,432,635 Stevens Oct. '17, 19222,578,412 Fisher Dec. 11, 1951 2,636,555 Klepetko et al Apr. 28, `19532,984,420* Hession May 16, 1961

1. IN A PROCESS FOR CONTINUOUSLY SPRAYING A LIQUIFORM AQUEOUS SUSPENSIONIN A CLOSED SYSTEM EVACUATED TO A LOW SUBATMOSPHERIC PRESSURE, THE STEPSCOMPRISING DIRECTING AT LEAST ONE JET STREAM OF SAID SUSPENSION INTO ANINTERMIXING ZONE IN COMMUNICATION WITH SAID CLOSED SYSTEM, DIRECTING ASTREAM OF SUPERHEATED STEAM INTO SAID INTERMIXING ZONE TO IMPINGE UPON,INTERMIX WITH AND HEAT SAID SUSPENSION STREAM, SAID STEAM BEINGSUPERHEATED TO A TEMPERATURE SUFFICIENT TO ELIMINATE FREEZING OF THESUSPENSION AT LEAST IN THE AREA OF INTRODUCTION INTO SAID CLOSED SYSTEMAND BEING INTRODUCED AT SUCH A RATE AS TO MAINTAIN A SUBATMOSPHERICZONAL VAPOR PRESSURE IN SAID INTERMIXING ZONE ABOVE THE PRESSURE IN SAIDCLOSED SYSTEM, AND DISCHARGING THE INTERMIXED STREAMS FROM SAIDINTERMIXING ZONE THROUGH A RESTRICTIVE ORIFICE ZONE INTO SAID CLOSEDSYSTEM TO MAINTAIN SAID VAPOR PRESSURE, SAID ZONAL VAPOR PRESSURETHEREBY PROPELLING AND DISPERSING SAID SUSPENSION INTO SAID CLOSEDSYSTEM TO FORM A SPRAY STREAM OF SUSPENSION DROPLETS IN WATER VAPORWHEREBY DRYING OF SAID DROPLETS IS MINIMIZED, WHEREIN SAID REDUCEDPRESSURE OF SAID CLOSED SYSTEM IS BELOW ABOUT 12 MM. HG, THE ZONAL VAPORPRESSURE IN SAID INTERMIXING ZONE IS OF THE ORDER OF ABOUT 150 MM. HGAND SAID STEAM IS SUPERHEATED TO A TEMPERATURE OF ABOUT 12 TO 20*F.ABOVE THE BOILING POINT IN SAID SYSTEM.